A serving is the amount of food recommended in consumer education materials such as MyPlate. A portion is the amount of a food you choose to eat at any one time â which may be more or less than a serving. Here are some everyday comparisons to help you figure out your serving sizes. 1  1 teaspoon of margarine is the size of one dice. 2  3 ounces of meat is the size of a deck of cards. 3  1 cup of pasta is the size of a baseball. 4  1Â½ ounces of cheese is the size of four stacked dice.
Oxygen: Specialized nerve cells within the aorta and carotid arteries called peripheral chemoreceptors monitor the oxygen concentration of the blood and feed back on the respiratory centers.
The central chemoreceptors do not directly detect the arterial CO2 tension. Instead, they detect decreases in the CSF pH. These two values are connected because arterial CO2 diffuses past the blood brain barrier, into the CSF, and is converted by carbonic anhydrase to carbonic acid that in turn decreases the CSF pH.